In a gas turbine burner a fuel is burned to produce hot pressurised exhaust gases which are then led to a turbine stage where they, while expanding and cooling, transfer momentum to turbine blades thereby imposing a rotational movement on a turbine rotor. Mechanical power of the turbine rotor can then be used to drive a generator for producing electrical power or to drive a machine. However, burning the fuel leads to a number of undesired pollutants in the exhaust gas which can cause damage to the environment. Therefore, it takes considerable effort to keep the pollutants as low as possible. One kind of pollutant is nitrous oxide (NOx). The rate of formation of nitrous oxide depends exponentially on the temperature of the combustion flame. It is therefore attempted to reduce the temperature over the combustion flame in order to keep the formation of nitrous oxide as low as possible.
There are two main measures by which reduction of the temperature of the combustion flame is achievable. The first is to use a lean stoichiometry, e.g. a fuel/air mixture with a low fuel fraction. The relatively small fraction of fuel leads to a combustion flame with a low temperature. The second measure is to provide a thorough mixing of fuel and air before the combustion takes place. The better the mixing is the more uniformly distributed is the fuel in the combustion zone. This helps to prevent hotspots in the combustion zone which would arise from local maxima in the fuel/air mixing ratio.
Modern gas turbine engines therefore use the concept of premixing air and fuel in lean stoichiometry before the combustion of the fuel/air mixture. Usually the pre-mixing takes place by injecting fuel into an air stream in a swirling zone of a combustor which is located upstream from the combustion zone. The swirling leads to a mixing of fuel and air before the mixture enters the combustion zone.
U.S. Pat. No. 6,513,329 B1 describes a premixing of fuel and air in a mixing chamber of a combustor. The mixing chamber extends along, and is at least partly wound around, a longitudinal axis of the burner. Two rows of fuel injection passages are located in the outer wall of the mixing chamber axis. The outlet opening of the mixing chamber is formed by slots extending parallel to the longitudinal burner axis. By this construction, the fuel/air mixture leaving the mixing chamber has, in addition to an axial streaming component with respect to the burner axis, a radial streaming component.
US 2001/0052229 A1 describes a burner with uniform fuel/air premixing for low emissions combustion. The burner comprises an air inlet duct and a swirler disposed in the air inlet duct. The swirler comprises swirler vanes with primary and secondary gas passages and corresponding gas inlet openings. Fuel flow through the two gas passages to the inlet openings is controlled independently, and enables control over the radial fuel/air concentration distribution profile from the swirler hub to the swirler trough. The secondary gas inlet openings are located downstream from the primary gas inlet openings.